On the validity of bias-extension test method for the characterisation of in-plane shear properties of rapid-cure prepregs

In-plane intraply shear deformation studies are commonly conducted as a means to characterise the formability of prepreg materials. Using observations and data collected during recent studies on rapid-cure epoxy thermoset prepregs, this study presents a critical analysis of widely-adopted test methods in this area. Particularly, the in-plane shear deformation behaviours of three different epoxy-carbon fibre prepregs with woven and unidirectional reinforcement architectures were characterised using bias-extension tests along with an optical strain measurement method. A range of industrially-relevant process parameters in terms of temperature and shear rate were used. It was demonstrated that under bias-extension deformation at higher temperatures, the experimentally measured shear angle deviates significantly from the theoretical, suggesting that inter-tow slippage becomes the dominant deformation mechanism. An attempt is made to characterise deviations in the case of each material using novel data visualisation methods. The validity of bias-extension testing without advanced instrumentation as a suitable means of characterisation of these materials at candidate process conditions is discussed. Finally, this investigation concludes to recommend an intermediate range of temperature to avoid a number of problems when processing rapid-cure prepregs.